Opinion

In a study published today,1 we demonstrate the statistical link between volcanic eruptions and measurements of deformation made from satellite radar. For this, we used a global synthesis of 18 years of measurements to calculate diagnostic test statistics that strongly associated satellite measurements of deformation with volcanic eruptions: 46% of deforming volcanoes erupted, while only 6% of non-deforming volcanoes (‘false negatives’) did.

These results demonstrate the usefulness of satellite deformation measurement (InSAR) for volcano monitoring and its potential for hazard assessments, especially in inaccessible settings. The launch of the European Space Agency’s first Sentinel satellite (expected in early April 2014) will increase the frequency of radar measurements, greatly improving the potential of InSAR measurements for volcano monitoring.

Why do we need to know about volcano deformation?

Volcano deformation, measurable from space, along with seismic activity and gas emissions, is an important indicator of a volcano’s level of activity: volcanoes deform before, during and after most eruptions.

A broad range of processes can cause deformation at volcanoes, including magma movement, gas escape, cooling and crystallisation and the settling of fresh deposits. Surface uplift is often interpreted as magma movement or increased pressure and is sometimes treated as an eruption precursor, although episodes of unrest do not necessarily end in eruption.

Traditional ground-based instruments for measuring deformation are only in operation at a small percentage of volcanoes worldwide, generally those that are near centres of population and have erupted recently. Satellite measurements of deformation have massively increased the number of volcanoes where deformation measurements can be made. At well-studied volcanoes, such as Etna or Kilauea, such measurements contribute to an understanding built up from a range of parameters over many decades. In remote or inaccessible regions, however, satellite deformation measurements over the last few years are often the only observations available. Understanding what they might mean in terms of volcanic hazard is therefore very important.

Why do we use satellite data?

A lot of what we know about volcanoes comes from detailed studies of a few, very active volcanoes. Furthermore, the time period over which we have been observing volcanoes captures only a very small fraction of their cycle of activity. Our ideas about indicators of unrest, like gas emission or deformation, are therefore biased towards volcanoes with frequent historical eruptions and brief repose periods. Because their coverage is global and observation repeat time is independent of levels of activity, satellite measurements have the potential to redress this bias.

However, for satellite data to be truly unbiased, measurements of volcano deformation must be made systematically, with both positive and `null’ results reported. Although some recent work has considered uncertainties for individual volcanoes and lack of deformation, where it is measured,2 the vast majority of published satellite deformation studies report only measurements of deformation. Overall, 59% of volcanoes covered by individual studies were shown to deform, relative to just 17% of those included in regional surveys.1

We therefore limited our synthesis to the 540 volcanoes covered by regional deformation studies. Our diagnostic test statistics were calculated for the subset of volcanoes (198) where measurements have been made over the full 18 years that satellite instruments have been available.

Interpreting satellite measurements of deformation

To calculate generally applicable statistics for what is a highly complicated dataset, we have made our classifications as simple as possible. For example, we classify each volcano in a systematically studied region as either ‘deforming’ or ‘not-deforming’ and either ‘erupting’ or ‘not erupting’ for a particular time period. We find that volcanoes that erupted between 1992 and 2010 were 4 times more likely to deform than not (positive likelihood ratio.)

The reality of deformation and eruption is of course much more complicated. Some volcanoes have multiple discrete episodes of deformation or eruption within a relatively brief time-period. Deformation can be associated closely with eruption, such as by pressurization of magma, or with processes not necessarily driven by volcanic activity, such as gravity-driven spreading. Satellite measurements may also miss highly localised, short-lived or low magnitude deformation.

Our classifications do not distinguish between deformation before, during or after eruption, and therefore do not imply a causal link. In order to test the usefulness of deformation as a precursor to volcanic eruption, rather than the association between deformation and eruption presented here, we need frequent measurements during all stages of a volcano’s eruptive cycle. New satellite instruments being launched in the coming months will improve temporal coverage to a level where this will be achievable for some volcanoes.

The causes and characteristics of deformation depend on tectonic settings, rock compositions and repose periods. The interpretation of satellite observations of deformation (or lack of deformation) should, therefore, take these factors into account. For example, a greater fraction of stratovolcanoes in arc settings erupt without satellite observations of deformation compared with shield volcanoes on rifts or hotspots. This has implications for the usefulness of satellite measurements for hazard assessment in different contexts.

By using a near-global dataset, our research draws out both regional differences in volcano deformation and a general association between deformation measurement and eruption.